One of the subsystems on a vehicle mounted internal combustion engine is the fuel system. In some forms of diesel engines, for instance, a primary fuel pump distributes fuel in timed relationship to the injectors, the amount of fuel so distributed being controlled by a fuel valve which is operated by linkages responsive to the normal throttle demand (accelerator pedal), the speed governor, and the air/fuel ratio regulator. However, the fuel is normally drawn from the tank and forced through suitable filters by a transfer pump, which may typically comprise a gear pump or the like as is known in the art, the pressure at the output of which is a substantial function of the speed of the pump. In order to provide fuel at desired ranges of pressures to the fuel valve of the primary pump, the transfer pump must have a sufficient capacity at low speeds to provide the required fuel for low speed operation; but since such pumps typically do not have a drooping pressure vs. speed characteristic, such a pump will provide fuel at too high a pressure when it is rotating at high speeds. Therefore, this type of fuel system has a pressure-regulating bypass valve which bypasses fuel back to the tank in sufficient quantity to limit the pressure of the fuel at the inlet to the primary pump fuel valve.
The operating capability of the engine is therefore directly dependent upon a healthy pressure-regulating bypass valve in the fuel system. In the past, it has been known to measure the pressure of fuel at the bypass valve with the engine running at low idle speed; this purports to provide a measure of bypass valve health. However, such a test only determines that the bypass valve is not stuck in the open position which would be indicated by a lower than normal fuel pressure at low idle speed; in fact, such a measurement does not isolate a fault to the bypass valve since, although it is unlikely, a bad transfer pump will also provide low pressure at low idle speed. Further, such a test does nothing to determine that the bypass valve is not stuck in the open position, which would simply provide normal pressure at low idle speed; and if the bypass valve is stuck in some position in between, a single test wouldn't indicate the particular problem, and may not even show up within the test limits utilized.